This invention relates to a control circuit for an AC power line; and more particularly to a timing control circuit for connecting a load to an AC power line for intermittent, selectively controlled time intervals.
This invention is concerned particularly with a control for a microwave oven, wherein the microwave generator is typically a magnetron. Since a magnetron cannot be operated efficiently or practically at varying power levels, the oven cannot be controlled by varying the line power to the magnetron. The oven may be controlled by applying power to the magnetron intermittently and cyclically, and by varying the times of the on-and-off portions of each power cycle. For control of a magnetron, the length of an on-off power cycle is referred to as the "time base". The time base of an oven control is selected and fixed by the manufacturer and may, for example, be as short as 1 second or as long as 30 seconds. The "duty cycle" of the control is the portion or percent of the time base cycle during which the magnetron is "on", and is selected by the user. The duty cycle may have a selectable range of from 25% to 100% of the time base.
While oven controls of this general type are known, because of the designed circuitry of the controls there may be considerable variation of either the time base or the duty cycle. It will be apparent that significant variations of either the time base or the duty cycle will have a considerable effect on either the cooking temperature or the cooking speed or both.
A principal object of this invention therefore is to provide a reliable and accurate control circuit, for variable control of a duty cycle during which AC power is applied to a load.
Another object of this invention is to provide such a circuit including means for calibration of the time base.
A further object of this invention is to provide such a control circuit which requires a minimum of components, resulting in ease and economy of manufacture.
Still another object of this invention is to provide such a circuit requiring minimum power for operation of the control, and which provides reliable wattage levels throughout the circuit for precision operation of the circuit.
A still further object of the invention is to provide such a control circuit which provides for precision control of duty cycle between 25% and 96% limits and also provides for 100% duty cycle operation.
These objects are accomplished in circuitry for variable control of an AC power circuit which includes the following subcircuits. A first timing circuit means is responsive to varying DC input control voltage levels for producing, alternately high and low DC output voltage levels; and includes input control means for comparing an input control voltage with upper and lower trigger voltage levels, for switching the circuit output between high and low voltage levels. A second power supply circuit means is coupled to the AC power circuit for producing a selected Vcc supply voltage for the first circuit means and other circuit means. A third synchronization circuit means is coupled to the AC power circuit for producing a DC threshold voltage level with superimposed negative-going half-wave pulses, synchronized with the AC power circuit; and this synchronization circuit means is coupled to the timing circuit means to provide the upper trigger voltage level for the timing circuit means. A fourth switching circuit means includes a DC operated electronic, alternating current switch for selectively opening and closing the AC power circuit. The operating circuit of the switch is connected between the positive side of a DC power supply and the output of the timing circuit. A fifth control voltage circuit means produces an input control voltage which increases or decreases at a selected rate. The input control voltage circuit means includes variable resistance means connected between the positive side of the DC power supply and the timing circuit input control means, and a capacitor connected between the timing circuit input control means and the negative side of the DC power supply. The variable resistance means includes a wiper contact connected to the timer input control means by a series connected diode which passes current to the input control means, and also connected to timing circuit grounding means. The timing circuit grounding means functions to connect the wiper contact to the negative side of the DC power supply when the timing circuit output voltage level is low.
The novel features and the advantages of the invention, as well as additional objects thereof, will be understood more fully from the following description when read in connection with the accompanying drawings.